The recording density of an optical storage device is dependent on the size of a light spot on the recording medium, which is formed by a light beam used for recording and reproducing. Recently, a reproducing method has been proposed, wherein it is possible to reproduce information recorded in domains whose size is smaller than the size of a light beam spot.
Normally, the light beam for use in optical recording is converged to a diffraction limit by a converging lens; therefore, the light intensity distribution shows a Gaussian distribution, and thus the temperature distribution on the recording medium also exhibits a virtual Gaussian distribution. As a result, an area having a temperature that is not less than a predetermined temperature becomes smaller in size than the size of the light beam spot. Consequently, the recording density can be greatly improved if only the spot having a temperature not less than the predetermined temperature is utilized for reproduction.
Referring to FIG. 5, the following description will discuss a magneto-optical disk wherein information recorded in domains whose size is smaller than the size of a light beam spot can be reproduced.
The magneto-optical disk is constituted of a substrate 21 as well as a readout layer 22 and a recording layer 23 formed on the substrate 21. The recording layer 23 has a great coercive force at room temperature. On the contrary, the readout layer 22 has a small coercive force at room temperature. When the temperature of an area of the readout layer 22 to be reproduced is raised, the direction of the magnetization thereon becomes coincident with the direction of the magnetization of the recording layer 23 due to the effect of the recording layer 23. That is, the magnetization of the recording layer 23 is copied onto the readout layer 22 by an exchange coupling force that is exerted between the readout layer 22 and the recording layer 23.
In the above arrangement, recording is executed by the ordinary photo-thermomagnetic recording method. When the recorded bits are to be reproduced, it is necessary to initialize the direction of magnetization of the readout layer 22 so as to make it coincident with the predetermined direction (upward in FIG. 5) by applying an auxiliary magnetic field from an auxiliary magnetic field generating device 24. Then, by projecting a reproduction-use light beam thereonto, the temperature of the recording layer 23 is locally raised and the magnetized information on the recording layer 23 is copied onto the readout layer 22. Thus, the temperature of a central portion of the area which has received the reproduction-use light beam is raised, and only the information located in the central portion is reproduced. Accordingly, recorded bits whose size is smaller than the size of the light beam spot are permitted to be read out.
However, in the above-mentioned conventional configuration, in the case of using a reproduction-use light beam with high intensity, a problem is presented in that, since the temperature of the adjoining recorded bits is also raised, the amplitude of a reproduced signal becomes smaller. On the contrary, in the case of using a reproduction-use light beam with low intensity, a problem is presented in that, since the temperature of a recorded bit is not raised, a reproduction signal can not be obtained.